How to tell if real 128k

It is most likely original. Does it have a SCSI port at the back? That was the most common of the few upgraded possible on this machine. That said, such an upgrade would not alter it's value too much. I'd say it would actually increase the value if it were a 512K. However if there are no obvious signs of an upgrade, then it's most likely mint

I'd value it around $2000 (seeing as you have the original mouse, keyboard and disks) based on the average of what I've seen them go for on eBay.

 

An actual original, first issue machine would simply say "Macintosh" on the back.

As for what is inside, it could be any of three possibilities, and a distant 4th.

Original Macintosh
Macintosh 512
Macintosh 512e (800K drive).

The 512e's 800K drive is pretty easy to spot just looking in the front. But an 800K drive can also be used in place of a 400K drive and you'd get an 800K MFS volume if you don't have the ROMs. It needs to be the MFD-51W-03 kind only. Possibly the newer ones may work, but the oldest ones do not.

If you have the ROM upgrade, you have 800K HFS and 400K MFS support. No 800K MFS is possible with the ROM upgrade.

The distant 4th possibility?

A General Computer hard drive installed, some kind of MacSnap, Levco or other RAM upgrade hack job installed. I can usually tell by weight/handling if there is something more inside a Mac case, and I generally will not power up any of these machines without first opening them. If there's some kind of Levco board in there, there's an excellent chance that it's come loose and will make your day if you turn things on.

 

Or

There is a fourth option. Looks like there were third party upgrades to upgrade the original 128 board to 512 without changing any thing else by adding an EPROM and bigger memory chips. I have two of those boards and no idea what company did it.

If you saw my previous post on my 128 you will see it says apple on the back and is textured not smooth. So it's a late model prototype or photo op machine as it looks like it was on the how to use this macintosh cover.

Value is relative but he would probably need the box and all extras to ask 2000 I would think.